Pulse echo recognition system



PULSE-'ECHO RECOGNITION SYSTEM Annu Elli I E' :E E

E, l' E: i' A A g WET f l l fsw/mwa V May 24, 1949. A, A, JOYNER 2,471,373

PULSE-ECHO RECOGNITION SYSTEM Filed sept. so, 1942 2 sheets-sheet 2 l L' f l In I .62 n i I i C 5 Z@ l. I l, i l@ i f l l l@ l i L 4|.i A-5 ZZL y l|:!:`fCf/QPIJL5E Jl't--ifdG/Wr/a/VPI/se Fg' .a

' 0 cejnncmor corresponding signals.

l enema May 24, vi949 l UNITED stares l einen PIULSE ECHO RECOGNITION SYSTEM rchibald A. Joyner, Merchantville, N. J., assigner to Radio Corporation of America, a corporation of Delaware September 30, 1942, Serial No. 460,291

4 Claims. (Cl. 343-13) Application This invention relates to pulse echo recognition systems and particularly to an improvement in pulse echo systems wherein recognition pulses and lecho pulses may be separately indicated and compared.

In a pulse echo system rpulses of energy are radiated toward visible and invisible aircraft or surface craft whichmay be operated by friendly or unfriendly hands. The pulses are reflected by such craft and received at or near the point of radiation. The received pulses are indicated by suitable means such as a cathode ray tube provided with means for deecting the ray as a. signal function and with additional means for deiiecting the ray in synchronism with the radiation of pulses. AThe indicator may be calibrated to indicate the pulse propagation time and hence the range or distance of the pulse reflecting objects.

Since it is normallyv impossible to determine from an echo signal itself if the reflected signal is from a friendly or unfriendly craft, it is desirable to provide some-means for challenging and recognizing the craft. There has previously been proposed a pulse echo system for indicating on a single cathode ray tube echo signals and recognition signals -whereby a plurality of echo signals may be compared to one or more identivfying signals.

In this previously proposed system the recognition signals may be delayed slightly with respect to the echo signals in order to obtain satisfactory retransmission from the craft.k As a result, the recognition and echo signals do not fall one directly below the other onthe cathode ray tube screen and there may be confusion in identifying lIt is thereforeone object of' the present inven- Ation to provide an improved method of and means Vfor .causing the recognition and echo pulses in a system of the above-described type to l appear invalignment on an indicator screen. A l further object of the invention is to provide an improved pulse echo recognitlonsystem. A still further object of the vinvention is to provide an improved method of and means for determining 4the time delay of electrical signals.

thereto duringeach interval that the recognition pulse axis is displaced. By changing the amplil tion of the tyfpe of echo and recognition signal indications that are obtained without employing my invention; Figure 3 is a circuit diagram of a portion of the system of Fig. l; Figure 4 is a graph showing incoming echo and recognition pulses, and Figs. 5 and 6 are diagrams that are referred to in explaining the invention. Similar reference characters will be applied to similar elementsin A the drawing.

'Referring to Fig. l, a pulse transmitter i, I

operated by a keyer 3 which is synchronized with a sweep generator 5, radiates pulses of carrier frequency (fl) toward an aircraft or wave refleeting object represented by the dash line box 1. If the aircraft is operated by friendly hands, it is provided with a suitable reectonor recognition transmitter, preferably operated in accordance with a prearrangeg secret code. The recognition transmitter mast include a pulse receiver 9 which is connected through a delay network I I to a pulse re-transmitter i3. The re-transmitter is preferably operated ata diiferent frequency (f2) from the frequency (Il of the original pulse transmitter. x.

The echo pulses from the craft are received on a pulse echo receiver l5 which is tuned to the pulse carrier frequency (fl). The retransmitted or recognition pulses are 'received on a re-transmission pulse receiver ll which is tuned tothe re-transmission 'pulse carrier frequency (f2) The outputs from the two receivers I5 and I'I are applied (as hereinafter described) -to a cathode ray tube I9 to which the sweep circuit generator voltages are also applied through a horizontal or sweep deflection amplifier 20.

The method of indicating the pulse -echo signals and the recognition signals is -asfollows: The sweep voltages are applied to deflect the cathode ray along a horizontal distance-scale. The

output voltages from the receivers liand l1 are applied aiternatelyto theverticaldeiiecti'ng electrodes. A bias voltage is appliedto the vernition signals. According to the present invention a bias is similarly applied to the horizontal deflecting plates to obtain the desired horizontal shift of the echo and recognition signals with respect to each other.

One circuit arrangement for applying the several voltages is as follows: A switching voltage is applied through a transformer 23 to asquare wave generator or trigger circuit 24 of the Eccles- Jordan type. Trigger circuit 24 comprises a thermionic tube 25 containing a pair of triode sections. The input and output circuits of the trigger circuit 24 are connected together in the usual manner so that first one, then the other tube passes anode current to form a voltage of square wave form.

The signal control grids 31, 33 of a pair of amplier tubes 33 and 35 are connected through blocking capacitors 4I, 43 to the output circuits of the reflected pulse receiver I and the pulse receiver I1, respectively. The anodes of the ampllflers 33, 35 are connected through blocking capacitors 32 and 34 to the vertical deflecting elements 45 and 45a of cathode ray tube I3. The anodes of the tube 25 are connected respectively through voltage divider resistors 25 and 21 to the grids 29, 3| of the amplifier tubes 33l 35. A bias voltage of square wave form is thus applied from the circuit 24 through the tubes 33, 35 to the vertical deflecting electrodes 45 and 45a of the cathode ray tube I3.

Another bias voltage of square Wave form is applied through a capacitor 55 and a conductor 6I to the horizontal deflection amplifier 23.

The pulse echo system proper'transmits pulses which are reflected by the aircraft 1 and are received by the receiver I5. -These echo pulses defleet the cathode ray vertically to produce echo pulse indications as shown in Fig. 2 during the time the trigger circuit 24 unblocks the amplifier 33. The square wave generator 24 simultaneously applies a bias to the deflecting plates 45 and 45a through the tubes 33 and 35 to move the time trace or axis 45 upwardly. The retransmitter operates after a slight delay (which prevents overloading the receiver 3 at the instant of reception) to transmit a recognition pulse. The re-transmission receiver I1 responds to the recognition signal which is applied to the cathode ray tube I3 through the amplifier 35 at the instant the trigger circuit 24 unblocks the amplier 35 and biases the cathode ray downwardly so that it moves along the lower time trace or axis 43.

It should be understood that the pulse rate is inversely proportional to the distances to be measured. The switching rate, at which the pulse echo signals and the recognition signals are Y applied, is preferably higher than the flicker rate.

Instead of using two receivers. a single receiver of the superheterodyne type may be used. The frequency of the oscillator of the superheterodyne may be shifted by reactance tubes controlled by the square wave voltages or two oscillators may be alternately keyed to make the receiver responsive to rst one frequency (fl) then the other frequency (f2) If lthe delay in the re-transmitter system is appreciable, the recognition signals will appear shifted slightly to the right as shown in Fig. 2. Provided similar delays are used in all the recognition systems, the shifting of the recognition signals may be overcome by including a phase shifter in either the echo pulse receiver or the echo transmitter (if a separate echo transmitter is used) so that the trace of each received pulse signal may be compared with the trace of the recognition signal immediately below.

However, it is preferred to utilize the present invention and overcome this shift of the recognition signals by mixing rectangular voltage pulses with the sweep voltage to produce horizontal displacement of the time axis, the amount of the displacement depending upon the amplitude of said pulses. The said pulses may be obtained from the trigger circuit 24 as shown in Fig. 1 where part of the square wave output is supplied from an anode of tube 2l through the blocking capacitor 55 to an amplitude control resistor 51 grounded at its lower end. A variable tap 53 on resistor 51 connects to the conductor 6I supplying the rectangular pulses to the ampliner 20 where they are added algebraically to the sweep voltages.

Fig. 3 shows the sweep deflection amplifier in more detail, this particular circuit being illustrated merely by way of example. A sawtooth voltage wave from the sweep generator 5 is applied over a conductor 52 and through a polarity reversing amplier 53 to the amplier tubes 55 and 51, respectively. A blocking capacitor and voltage divider resistors are indicated at 53, 1I and 13, respectively. The amplified sawtooth voltage is applied in push-pull relation through' blocking capacitors 15 and 11 to the horizontal or sweep deflecting plates 13 and 13a of the cathode ray tube I3. A centering voltage may be applied to the plates 13 and 13a through resistors 3l and 33.

The square, wave voltage that is supplied over conductor 6I for shifting the pulse indications to the right or to the left on the time or sweep axis may be applied to the control grid of tube 51. Thus, during the positive half cycle of the square wave the echo signal is shifted along the time axis 46 slightly to the right and during the negative half cycle the recognition pulses are shifted along the time axisl 43 slightly to the left. The amount of shift is determined by the setting of the amplitude conrol tap 53 (Fig. 1) whereby the recognition pulses on axis 43 may be brought directly underneath the echo pulses on the axis 45. This action is illustrated in Figs. 5 and 6 where Fig. 5 shows echo and recognition pulses as they appear on the cathode ray tube screen before the square wave delay correction voltage has been added, assuming the received recognition pulses have been delayed with respect to the echo pulses as indicated in Fig. 4. The addition of the square wave to the horizontal deflection sawtooth wave aligns the echo and recognition pulses as shown in Fig. 6 when the square wave is adjusted to the proper amplitude.

My invention may be utilized, if desired, generally for the purpose of measuring the time delay between electrical pulses or other signals. For example, referring to Fig. 1 the adjustable tap 53 may be provided with an index member or pointer and an associated scale that is calibrated in time delay units. Since a certain amplitude adjustment of the square wave shifts the two signals a definite amount with respect to each other, the scale reading gives the delay between pulses if the two signals have been brought into alignment or if they have been superimposed in case the two time axes are not displaced.

A similar arrangement may ,be utilized in a recognition system to makeA more certain that recognition signals havenot been transmitted from an enemy aircraft. The amount of delay provided |by the delay network Il in the aircraft apparatus may be made adjustable so that the delay has diierent predetermined values at prescribed intervals during the day. Then even if the enemy learns the code (but not the delay setting schedule) and responds to a challenge, it will be known that it is an enemy aircraft because the time delay of the recognition signals will be wrong.

I claim as my invention:

1. The method of determining the time delay between two electrical pulses, said method comprising .producing an electron beam, defiecting said beam successively along a time axis with the duration of each sweep along the time axis llongcompared with the duration of each of said pulses, modulating said beam by one of said pulses during one deection to produce an indication and by the other of saidv pulses during a succeeding deflection to produce a second indication, shifting the beam deection in a certain direction along its time axis during the time that the beam is modulatedby at least one of said,

signals and by an-amount such as to obtain coincidence of said indications on said time axis whereby the amount of said shift indicates the along its time axis during the time that the beam is being modulated by'one group of said pulses only, and shifting the beam deiiectio'nv in the opposite direction along its time axis during the time that the beam is being modulated bythe other group of said pulses only, until'said shift brings the pulse indications of said groups of pulses into coincidence on said time axis.

3. In combination, a cathode ray tube having a screen and means for directing a cathode beam against said screen, means for deecting said beam along a time axis, means for periodically displacing said time axis, a plurality ofsignal sources, means for modulating said'beam by one signal source while said deflectionis along one time axis position to produce signal marks on the' screen, means for modulating said beam by a different signal source while said deilection is along a different time axis position to yproduce other signal marks on the screen, and means for displacing said first signal marks and said other signal marks with respect to each otheriin the direction of their time axes.

4. A pulse echo and pulse recognition system including means for radiating pulses of energy, means for receiving said radiated pulses after reection from a craft remote from said receiving means andsubject to challenge, cathode ray tube means connected to said receiving means for indicating along a time axis said received' means for shifting the positions ofthe rst and' second pulse indications with respect to each other in the direction of the time axis to correct for displacement-.introduced by said delay in retransmission. ARCHIBALD A. VJOYNER.

REFERENCES CITED The following' references are of record'in the le of this patent: e

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